Despite widespread use of urine cannabinoid assays in drug treatment, workplace monitoring, and forensic toxicology programs, limited urinary cannabinoid excretion data are available in the literature to aid in the interpretation of test results. Much of the available published information is based on the use of non-specific immunoassay tests that have dramatically changed performance characteristics over time. Also, both the timeframe of specimen collection and the number of specimens analyzed in these studies have been restricted. The urinary excretion kinetics of smoked marijuana have been characterized utilizing GC/MS urine cannabinoid data collected in this study. Substantial intersubject variability was noted between subjects and between doses in patterns of 11-nor-9-carboxy-D9-tetrahydrocannabinol (THCCOOH) excretion. Mean peak urine THCCOOH concentrations averaged 89.8 +/- 31.9 ng/mL and 153.4 +/- 49.2 ng/mL following smoking of approximately 15.8 mg and 33.8 mg THC, respectively. The mean times of peak urine concentration were 7.7 h +/- 0.8 after the 1.75% THC and 13.9 h +/- 3.5 after the 3.55% THC dose. Mean GC/MS THCCOOH detection times for the last positive urine sample following the smoking of a single 1.75% or 3.55% THC cigarette were 33.7 +/- 9.2 h and 88.6 +/- 9.5 h, respectively, when a 15 ng/mL cutoff concentration was utilized. Although it is known that the excretion of marijuana metabolites occurs over an extended period of time, few studies have been designed to accurately estimate excretion half-lives. We monitored excretion of the primary urinary metabolite of marijuana, THCCOOH, by GC/MS. Terminal elimination half-lives of THCCOOH were determined in six male subjects following smoking of a single 1.75% or 3.55% THC cigarette by both the amount remaining to be excreted (ARE) or sigma minus method and by the excretion rate method. Mean +/- SEM half-lives calculated by the ARE method following the low or high dose were 31.4 +/- 1.1 h (range 27.8 to 35.3) and 28.6 +/- 1.5 h (range 24.9 to 34.5), respectively. The amounts of THCCOOH excreted over a seven day period were 93.9 +/- 24.5 ug (range 34.6 to 171.6) and 197.6 +/- 33.7 ug. This study documents the prolonged excretion of THCCOOH in urine and emphasizes the importance of study design in precise estimation of terminal elimination half-lives. A sensitive analytical method and a one to two week specimen collection period are important study considerations in the monitoring of marijuana excretion.